Large field of view is a critical parameter for the implementation of many optical systems. For example, practical implementations of high spectral resolution space based optical systems, such as direct detection Doppler wind lidars, require a receiver field of view (FOV) large enough to encompass a laser beam of sufficient divergence as to meet near surface eye-safety requirements, and allow for reasonable thermal wander between the receiver and transmitter optic axes. Typically, this receiver FOV is on the order of 100 microradians. Furthermore, to acquire sufficient signal from orbit, large collection optics on the order of 1 meter diameter are required. To minimize mass, volume, and cost of the interferometer components, the large diameter beam from the collection telescope is typically recollimated to the smaller diameter of practical interferometer optics, typically 50 mm or less. Because the etendu or optical throughput of optical systems is conserved, the product of the beam diameter and its divergence remains a fixed property of the system. Consequently, a magnified field angle (order of several milliradians) is experienced within the interferometer. In general, the spectral or phase resolution of interferometers is dependent on the angular divergence of the light propagating within the interferometric path because rays of different angles traverse different path lengths producing wavefront or phase errors. Therefore, as field of view of the system increases, the spectral and phase resolution of the system decrease.
Techniques for widening the operative field of view of optical devices have been developed and proposed. For example, field widened interferometers have used combinations of different types of glass to compensate interferometer path length changes with field angle. However, the use of large blocks of glass results in the absorption and scattering of optical signals, limiting operational wavelengths. In addition, the blocks of glass typically have limited index of refraction homogeneity, affecting intra-interferometer wavefront and resolution. In addition, the use of large blocks of glass implies increased mass, which is a particular disadvantage in connection with optical systems intended for airborne or space borne deployment. Accordingly, while such refractive compensation can be effective at improving contrast for larger field of view instruments, they are incapable of improving the effect of wavefront error, and can in fact increase such error. In addition, for large optical path differences, the refractive method requires adding large amounts of refractive material, which can itself reduce contrast because of refractive index inhomogeneities, in some cases due to thermal gradients in the large glass blocks. Thus, in large optical path difference interferometers (i.e. high spectral and phase resolution), the refractive method compounds the effect of poor wavefront error, since the wavefront from short and long arms can be substantially different, resulting in low contrast fringes. In addition, refractive solutions are not practical at ultraviolet wavelengths, where glass absorption is high, and such solutions complicate multiple simultaneous wavelength operation due to dispersion.
Cat's eye mirrors that alter the optical path length traversed by optical rays across the field of view of an instrument have been made. However, these systems have an obscuration when implemented as part of a Michelson interferometer, and, by symmetry, cannot use phase mask, or phase mirror technology that is essential for high speed applications. Moreover, the obscuration means that any sufficiently complicated wavefront will not be invariant (i.e., the input and output will not be identical). This is a particularly important effect if the input light is from a multimode optical fiber, where the wavefront is extremely complicated and where any diffraction losses, such as occur at an obscuration, quickly alter the shape and phase of the wavefront. In addition, tolerance to front end optical wavefront errors is particularly important in controlling the cost and mass of signal collection and routing optics.
A standard (flat mirror) Michelson or Mach-Zehnder interferometer works well with larger fields of view and significant wavefront error only if the optical path difference within the instrument is small. With moderately large optical path difference (for increased spectral and phase sensitivity), as the field of view increases or the incident wavefront worsens, the interferogram contrast degrades. The primary problem has been increasing the optical path difference while maintaining a moderately high field of view, without the need for an input signal with a high quality wavefront. Other problems include simultaneous or sequential operation at multiple wavelengths and at high speed to accommodate usual lidar (light detection and ranging) return signals. Still another problem is calibration of the interferometer at a high data rate without introducing additional losses.
Accordingly, it would be desirable to provide an interferometer that is spectrally sensitive while simultaneously operating at high speed, with a moderate field of view, and at multiple wavelengths, with possibly poor wavefront quality or a highly speckled multi-mode fiber input. It would also be desirable to provide optical systems, including interferometers that provide high contrast and resolution even with a large field of view in single pixel and imaging systems.